1. Field of the Invention
The present invention relates to a process for coloring plastics and the plastics prepared thereby. More particularly, the present invention concerns the use of iron oxide spinels which are stable at high temperatures to color plastics.
2. Prior Art
As is known to those skilled in the art to which the present invention pertains, most inorganic pigments are not suitable for plastics processing because of their thermal instability at temperatures above 175.degree. C. Thus, the known yellow pigments such as iron oxide, lead chromate and zinc chromate are not well chosen for the yellow coloring of plastics such as polyethylene, polypropylene, polyvinyl chloride, polycarbonates, polyamide and the like. The same is true of other inorganic pigments for obtaining other colors. Furthermore, organic pigments show color degradation at temperatures of about 175.degree. C., thereby precluding their use in the processing of plastics. Thus the need for pigments stable at high temperatures and capable of giving various desired colors is readily apparent.
Applicants' copending applications referred to above and which are incorporated by reference into this application, describe a novel form a iron oxide spinels. There was previously no method known of producing the subject spinels. The spinels are useful in coloring plastics because they are acicular particles having an average length of less than 0.1 micron and are stable at high temperatures.